1. Field of the Invention
The present invention relates to fasteners and, more specifically to a fastening system including a multi-piece, swage-type fastener and a swage tool therefore, which exhibit an optimum balance of low swage load and high strength. The invention also relates to a simplified installation method using a low swage load fastening system.
2. Description of the Related Art
In many commercial applications, two-piece threaded or swaged fasteners, commonly referred to as lockbolts, are commonly used to secure a number of workpieces together. See, e.g., U.S. Pat. Nos. 2,531,048; 3,215,024; 3,915,053; 4,472,096; and 5,090,852. The material properties (e.g., without limitation, tensile strength and hardness) of these fasteners varies depending on the commercial application in which the fasteners will be used. To distinguish the varying properties of fasteners, the fasteners are typically designated by Grade. The Grade of a fastener is indicative of its strength. Industry standards establish the requisite strength of a fastener in order to meet a particular Grade, with the strength of a particular fastener being determined by the strength of the material of the fastener bolt or pin. For example, a ½ inch Grade 5 fastener has a ½ inch diameter pin or bolt shank portion for use in a nominal ½ inch diameter workpiece opening and, in accordance with SAE J429, Grade 5 or ASTM A-325, such Grade 5 fastener must have a minimum tensile strength of 120 KSI. By way of comparison, in order to qualify as a Grade 8 fastener, per SAE J429, Grade 8 or ASTM A-490, the fastener must have a minimum tensile strength of 150 KSI. Grade 5 fasteners are often used, for example, in railroad (e.g., railcar) applications. Grade 8 fasteners are commonly employed in commercial transportation applications, for example, to secure truck components within the commercial trucking industry.
Typically swage-type fasteners include a pin and a collar. Most of these fasteners are of the pull-type variety and include a pin shank with a locking portion having lock grooves and a pull portion having pull grooves. The pull grooves are adapted to be gripped by matching teeth in chuck jaws of an installation tool with a swage anvil. The swage anvil is adapted to engage the collar and apply a relative axial force between the pin and collar, and to move over the collar and swage it into the lock grooves. The relative axial force comprises a combination of the tensile load on the pin caused by the chuck jaws and the compressive load on the collar caused by the swage anvil. The pull portion of many swage-type fasteners is connected to the lock groove portion by a breakneck groove of reduced strength. The breakneck groove is adapted to fracture at a preselected magnitude of axial or tensile force which is greater than that required to swage the collar. Accordingly, the pull portion, or pintail, will be severed and removed from the pin shank after completion of swaging. Other swage fasteners, however, have pull portions which remain on the pin after completion of installation. See, e.g., U.S. Pat. Nos. 5,315,755, 5,548,889 and 5,604,968 (disclosing a threaded pull portion which is not severed from the pin). In other words, these fasteners are pintail-less. See, e.g., FIGS. 1-8 of the '755 patent.
Among the problems frequently encountered with swage-type fasteners of relatively high strength (e.g., Grade 5 and above), is the excessive magnitude of applied tensile load required in order to fully swage the collar. This results in premature wear of the installation tool, particularly the pulling mechanism, and also stripping of the pull grooves on the pin. The high swage load also complicates the installation process in general, especially where manually operated installation tools are used. In an attempt to overcome some of these disadvantages, a variety of different installation tool modifications have been made. For example, U.S. Pat. No. 4,299,519, which is hereby incorporated herein by reference as if fully set forth herein, discloses an acorn-shaped pin pull portion and complimentary shaped tool gripping structure which are intended to provide engagement of all of the pull grooves of the pull portion by the gripping structure of the tool and, thereby resist stripping of the pull grooves. See, e.g., FIGS. 1-5 of the '519 patent. Other tools merely incorporate hydraulic and/or pneumatic piston-cylinders, in order to assist in applying the necessary swaging force. See, e.g., U.S. Pat. Nos. 4,597,263 and 4,878,372. However, this adds size and weight to the tool, which can make it awkward to handle or limit accessibility thereby potentially jeopardizing its precise application to drive the fastener and thus the quality of the final installation. Accordingly, there is room for improvement of the installation tool and method for swage-type fastening systems.
The high swage load is largely the result of the use of fastener collars which have an increased wall thickness and are thus structured to overfill or overpack the lock grooves in order to achieve the requisite strength, for a particular fastener Grade (e.g., Grade 5 and Grade 8). See, e.g., U.S. Pat. No. 5,090,852, supra, (disclosing a modified pin thread form to include shallow grooves and a streamlined root contour, with the collar having an increased wall thickness with sufficient material to overpack such shallow grooves, in order to achieve the requisite shear strength); see also the '755 patent, supra, and U.S. Pat. Nos. 5,548,889 and 5,604,968 (disclosing a shallow pin groove construction and a collar shank with a volume having an excess of material of at least 16% in order to overpack the grooves). Overpacking of the lock grooves is problematic. It is a primary source of the aforementioned undesirable excessively high swage load. Accordingly, several attempts have been made, through variations in swage-type fastener design, to combat these disadvantages and, in particular, the prohibitively high swage load.
For example, U.S. Pat. Nos. 6,233,802 and 6,497,024, which are hereby incorporated herein by reference as if fully set forth herein, disclose a fastening system for a two-piece, swage-type fastener having a wide, shallow lock groove thread form which is designed to permit the fastener to be installed at a lower swage load than a conventional swage-type fastener of comparable Grade yet retain essentially the same physical properties (e.g., tensile strength; clamp load) when installed. The lower swage load permits the advantageous use of a smaller, lighter weight installation tool. The system is described as applied to fasteners having pintails as well as pintail-less fasteners. For pull-type swage fasteners having threaded pull portions which are not severed, as in the '755 patent, supra, the invention is stated to facilitate the engagement of fewer threads on the pull portion because the extra force required to fracture a breakneck, is not required. This is said to (1) result in less stress on the engaged threads of the mating threaded thimble or nut member of the pull tool, thereby extending tool life; (2) permit the use of a shorter, less expensive pin because less pin protrusion is required in order to grip the reduced number of pull grooves which must be gripped; and (3) permit the installation tool to be smaller and thus lighter and less expensive, because lower applied loads are required for final installation. The system also facilitates the use of an internal drive. See, e.g., FIGS. 17 and 18 of the '755 patent (illustrating an internal drive including a threaded pull rod or spindle engageable within the pull grooves of a threaded bore in the end of the pin shank). As described in the '755 patent and U.S. Pat. Nos. 5,548,889 and 5,604,968, which are hereby incorporated herein by reference as if fully set forth herein, use of such internal drive allows for a reduction in protrusion relative to the external drive which, in turn, provides for a more efficient final fit and appearance of the fastener.
However, as disclosed in the '024 patent, supra, in order to achieve the foregoing benefits, it was necessary to change the thread form (e.g., lock groove structure) to a wider and shallower configuration. Although revising the thread form is a viable option to reduce swage load, the thread form disclosed in the '024 patent is a drastic change requiring significantly larger pitch and a plurality of different radii with a relatively abrupt and discontinuous transition among the radii (e.g. from one radius to another). Producing pins with the disclosed thread form is, therefore, difficult and cost intensive. Additionally, the discontinuous transition among radii of the thread form prevents maximizing the complimentary engagement of the thread form by the collar grooves, when the fastener is swaged. There, therefore, is also room for further improvement in the thread form for pin lock grooves.
Additionally, collars of the type disclosed in the '802 and '024 patents have a very narrow range of acceptable hardness, because too much hardness results in a prohibitively high swage load and too little hardness has insufficient strength. This is particularly problematic with respect to higher Grade fasteners (e.g., Grade 5 and above) with which an increase in hardness of both the bolt and collar is required in order to meet industry tensile strength requirements. Accordingly, known collars must be subjected to a thermal process in order to be soft enough to swage and be compatible with the revised thread form, but strong enough to meet industry Grade standards. This adds still further cost and complexity to the manufacture of the fastener. For example, two such thermal processing methods include stress relieving and quench and tempering; obtaining consistent desired hardness by stress relieving is very difficult to accomplish and quench and tempering is expensive and difficult to accomplish without undesirably carburizing or decarburizing the surface of the collars. Both methods are time and cost intensive, requiring, for example, the added expense of furnace operating costs.
It is, therefore, highly desirable to provide a high-strength, low swage load fastening system which exhibits, among other attributes, all of the benefits of the low swage fasteners disclosed in the '519, '755, '802, and '024 patents, supra, but which, also does not require, for example, cost and time intensive thermal processing of the collar.
There is, therefore, room for improvement in the art of high-strength, low swage load fastening systems.